1. Field of the Invention
The present invention relates to a lens system which is corrected for chromatic aberration with respect to two or three wavelengths. The system with two or three colors.
Description of the Background Art
Transferring patterns photo mask to a photoresist film is one of the processes in fabricating a semiconductor device The pattern transferring process includes: coating a semiconductor substrate with photoresist material: and illuminating with light the semiconductor substrate through a photomask having a predetermined mask pattern, to thereby expose the photoresist film. (The exposing step is referred to as "transfer treatment".) Normally, for production of a single semiconductor device, it is necessary to prepare various types of photomasks and to transfer the pattern for each photomask. In the transfer treatment, therefore, the photomask must be aligned so that the pattern of the photomask is matched with a pattern previously formed on the semiconductor substrate with a predetermined relationship. Conventionally the alignment has been carried out by visual observation through a microscope.
In general, photoresist material is highly sensitive to light with short wavelength but is not sensitive to light with a long wavelength. Accordingly, light with a long wavelength (for example, e-line) is used for the alignment, and light with a short wavelength (for example, g-line) is used for the transfer treatment.
For this reason, it is desired to use a lens system which is corrected for chromatic aberration with respect to two wavelengths, i.e., a long wavelength and a short wavelength A large number of lens systems which are corrected for chromatic aberration with respect to two wavelengths have been conventionally proposed.
In the prior art, however, there has been no lens system in which average image points with respect to the respective wavelengths substantially coincide with each other and wherein the image fields with respect to the respective wavelengths substantially coincide with each other within the range from the vicinity of an optical axis to the image peripheral portion. When, for example, the image fields with respect to the respective wavelengths are registered with each other in the vicinity of the optical axis while not being registered with each other in the image peripheral portion, the problem described below occurs. In the vicinity of the optical axis, the transfer treatment may be performed preferably without the adjustment of focus so long as the foucing operation is carried out in the alignment. In the image peripheral portion, on the contrary, the focal point in the alignment is deviated from that in the transfer treatment. Unless either the photomask or the semiconductor substrate moves along the optical axis in order to be in focus, therefore, the transfer treatment cannot be performed at a preferred resolution.
For reduction of exposure time and improvement in operating efficiency, it is preferable to use light having a plurality of rays wherein the wavelengths thereof are different from each other. For example, exposing of a photoresist film using the g- and h-lines is more favorable than exposing using only the g-line. In this case, for the transfer treatment with two different short wavelengths Such as the g- and h-lines, the lens system must be corrected for the chromatic aberration at least with respect to the g- and h-lines.
For eliminating the focusing operation in the alignment process and reducing the operating time for the transfer treatment, it is required to design a lens system which is corrected for chromatic aberration with respect to the long wavelength used in aligning and with respect to two different short wavelengths used in exposing. Furthermore, in order to transfer the whole image of the photomask to the photoresist film at a preferred resolution, it is required that the average image points with respect to the respective wavelengths substantially coincide with each other and that the image fields also coincide with each other within the range from the vicinity of the optical axis to the image peripheral portion. However, no lens system which meets these requirements has been heretofore proposed.